NFS: remove l_pid field from nfs_lockowner

this field is not used in any important way and probably should
have been removed by

Commit: 8003d3c4

 ("nfs4: treat lock owners as opaque values")

which removed the pid argument from nfs4_get_lock_state.

Except in unusual and uninteresting cases, two threads with the same
->tgid will have the same ->files pointer, so keeping them both
for comparison brings no benefit.

Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>